Feedstocks containing higher molecular weight hydrocarbons are cracked by contacting the feedstocks under elevated temperatures with a cracking catalyst whereby light and middle distillates are produced. Typically, the octane number of the light distillate (gasoline) is dependent upon the riser temperature, conversion level of operation or the catalyst type. Therefore, to increase the octane number of the gasoline, conversion of the hydrocarbon feed to lighter products must be increased by preferably raising the temperature of operation, or by increasing other operating variables such as catalyst to oil ratio. Unfortunately, a limit on the maximum operating temperature is set by reactor metallurgy, gas compressor constraint or other operating constraints. Increasing conversion by other means may also result in poor selectivity to desired products. The octane number of the gasoline may be increased by switching from a catalyst containing rare earth-exchanged Y zeolite to one containing ultrastable Y zeolite or ZSM-5, as is well known in prior art; however, such a switch will generally involve substantially higher costs, be time consuming, and above all, lead to significant reductions in the yield of gasoline.
U.S. Pat. No. 4,869,807 teaches that a desirable way to advantageously increase the octane number of the gasoline produced in the process is to charge some of the fresh hydrocarbon feed to upper injection points along the length of the riser while charging a majority of the fresh feed to the bottom of the riser.
One problem with the process as taught in U.S. Pat. No. 4,869,807 is that an undesirable increase in C.sub.2 - gas make accompanies the desirable increase in gasoline octanes (see Examples I, II and III, Tables II, III and IV in U.S. Pat. No. 4,869,807).
Therefore, it is desirable to have a modified cracking process available for increasing the octane number of the gasoline while minimizing the disadvantages associated with practices described in the prior art.
It is thus one object of this invention to provide a regenerable fluid catalytic cracking process, and a further object of this invention to provide a process for increasing the octane number of the gasoline from the process. Another object of this invention is to achieve the increase in octane number of the gasoline while minimizing undesirable gas make, or coke make, by modifying the method of introduction of feed to the riser reactor in a fluid catalytic cracking process.